Programming for Mobile Computing

Transcription

1 1/57 Programming for Mobile Computing EECS 1022 moodle.yorku.ca

2 Labs 2/57 For the things we have to learn before we can do them, we learn by doing them. Aristotle During the labs, carefully read the instructions, try to follow them, consult the lecture slides, but also experiment and ask the teaching assistants for help. Work in teams of two as you can learn from each other.

3 Labs 3/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name.

4 Labs 3/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. What is the type of the attribute?

5 Labs 3/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. What is the type of the attribute? Answer int.

6 Labs 3/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. What is the type of the attribute? Answer int. What is a descriptive name for the attribute?

7 Labs 3/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. What is the type of the attribute? Answer int. What is a descriptive name for the attribute? Answer counter.

8 Labs 4/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. How do you declare an attribute?

9 Labs 4/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. How do you declare an attribute? Answer private type name;

10 Labs 4/57 Problem An attribute that represents a counter that counts the number of times any of the four buttons is pressed. Use a descriptive name. How do you declare an attribute? Answer private type name; Solution private int counter;

11 Today s Lecture 5/57 Classes Objects

12 Data types: definition 6/57 A data type consists of a name, a set of values, and a set of operations.

13 Data types: example 7/57 name: int values: 0, 1, 1, 2, 2,... operations: +, -, *, /,...

14 How many values of type int are there? 8/57 How many bytes are used to represent a value of type int?

15 How many values of type int are there? 8/57 How many bytes are used to represent a value of type int? Answer 4

16 How many values of type int are there? 8/57 How many bytes are used to represent a value of type int? Answer 4 A byte consists of how many bits?

17 How many values of type int are there? 8/57 How many bytes are used to represent a value of type int? Answer 4 A byte consists of how many bits? Answer 8

18 How many values of type int are there? 9/57 How many bits are used to represent a value of type int?

19 How many values of type int are there? 9/57 How many bits are used to represent a value of type int? Answer 4 8 = 32

20 How many values of type int are there? 9/57 How many bits are used to represent a value of type int? Answer 4 8 = 32 How many different values has a bit?

21 How many values of type int are there? 9/57 How many bits are used to represent a value of type int? Answer 4 8 = 32 How many different values has a bit? Answer 2

22 How many values of type int are there? 10/57 How many values of type int are there?

23 How many values of type int are there? 10/57 How many values of type int are there? Answer } 2 2 {{ 2 } = times

24 Data types: example 11/57 name: int values: [ , ] operations: +, -, *, /,... Note that = 2 31.

25 Operations 12/57 The operations are typed. For example, : (int int) int specifies that the operation takes two values of type int and returns a value of type int.

26 Data types: example 13/57 name: int values: [ , ] operations: + : (int int) int : (int int) int : (int int) int / : (int int) int...

27 Data types: example 14/57 name: boolean values: true, false operations: && : (boolean boolean) boolean : (boolean boolean) boolean! : boolean boolean

28 Primitive types 15/57 What are the names of the five most used primitive types?

29 Primitive types 15/57 What are the names of the five most used primitive types? Answer boolean, char, double, int and long. a a The other three, less used, primitive types are byte, float and short.

30 Location of the robot 16/57 Can the location of the robot be represented as a single value of primitive type?

31 Location of the robot 16/57 Can the location of the robot be represented as a single value of primitive type? Answer No.

32 Programming Paradigms 17/57 Object-oriented programming Imperative programming Functional programming Logic programming Concurrent programming Event-driven programming Constraint programming...

33 Object-Oriented Programming 18/57 Objects as a formal concept in programming were introduced in the 1960s in programming language Simula 67. This language was created by Ole-Johan Dahl and Kristen Nygaard of the Norwegian Computing Center in Oslo.

34 Ole-Johan Dahl 19/57 Ole-Johan Dahl (October 12, 1931 June 29, 2002) was a Norwegian computer scientist and is considered to be one of the fathers of object-oriented programming. source: ifi.uio.no

35 Kristen Nygaard 20/57 Kristen Nygaard (August 27, 1926 August 10, 2002) was a Norwegian computer scientist and is considered to be one of the fathers of objectoriented programming. source: ifi.uio.no

36 Dahl and Nygaard 21/57 In 2001, Ole-Johan Dahl and Kristen Nygaard won the Turing award. The A.M. Turing Award is given annually by the Association for Computing Machinery (ACM) to an individual selected for contributions of a technical nature made to the computing community. The Turing Award is recognized as the highest distinction in Computer Science and Nobel Prize of computing. source: ifi.uio.no

37 Advantages of OOP 22/57 easy to re-use code easy to extend code easy to maintain code easy to test code fits well with the real world... However, (some of) these advantages are debatable. Mordechai Ben-Ari. Objects never?: well, hardly ever! Communications of the ACM, 53(9): 32 35, September 2010.

38 Location of the robot 23/57 How can we represent the location of the robot? (You may use multiple values of primitive type.)

39 Location of the robot 23/57 How can we represent the location of the robot? (You may use multiple values of primitive type.) Answer Its x- and y-coordinate.

40 Location of the robot 23/57 How can we represent the location of the robot? (You may use multiple values of primitive type.) Answer Its x- and y-coordinate. For each datum, what is a descriptive name and an appropriate type?

41 Location of the robot 23/57 How can we represent the location of the robot? (You may use multiple values of primitive type.) Answer Its x- and y-coordinate. For each datum, what is a descriptive name and an appropriate type? Answer x : int y : int

42 How to represent locations? 24/57 How to represent the location (1, 2)?

43 How to represent locations? 24/57 How to represent the location (1, 2)? Answer x 1 y 2

44 How to represent locations? 24/57 How to represent the location (1, 2)? Answer x 1 y 2 How to represent the location (3, 4)?

45 How to represent locations? 24/57 How to represent the location (1, 2)? Answer x 1 y 2 How to represent the location (3, 4)? Answer x 3 y 4

46 How to represent locations? 25/57 All locations are an instance of the following pattern. x y

47 How to manipulate locations? 26/57 If you are given an instance of the pattern x y what kind of questions may you want to ask about this data?

48 How to manipulate locations? 26/57 If you are given an instance of the pattern x y what kind of questions may you want to ask about this data? What is the x-coordinate of this location?

49 How to manipulate locations? 26/57 If you are given an instance of the pattern x y what kind of questions may you want to ask about this data? What is the x-coordinate of this location? What is the y-coordinate of this location?

50 How to manipulate locations? 26/57 If you are given an instance of the pattern x y what kind of questions may you want to ask about this data? What is the x-coordinate of this location? What is the y-coordinate of this location? How does the location change if the robot moves North?

51 How to manipulate locations? 26/57 If you are given an instance of the pattern x y what kind of questions may you want to ask about this data? What is the x-coordinate of this location? What is the y-coordinate of this location? How does the location change if the robot moves North? How does the location change if the robot moves South?...

52 Objects and classes 27/57 What is an object? Answer An instance of a class. What is a class? Answer A blueprint for objects. You often find these circular definitions in textbooks and on the Internet, but they are not particularly helpful.

53 What is a class? 28/57 x y A class contains attributes. Each attribute has a name and a type. x : int y : int

54 What is a class? 29/57 What is the x-coordinate of this location? What is the y-coordinate of this location?... A class contains methods. Each method has a signature and possibly a return type. getx() : int gety() : int

55 What is an object? 30/57 An object is an instance of a class. An object has a state. The state of an object consists of the attributes of the class and their values. x 1 y 2

56 What is an object? 31/57 An object has an identity. This identity is unique. That is, two different objects have different identities. This is an abstract notion. In more concrete terms, you may think of an object s identity as the address in memory where it is stored. Obviously, two different objects cannot be stored at the same memory address.

57 How to create objects? 32/57 int a = 1; int b = 2; Location location = new Location(a, b);

58 How to create objects? 33/57 int a = 1; int b = 2; Location location = new Location(a, b); 0. 8 a b location.

59 How to create objects? 34/57 int a = 1; int b = 2; Location location = new Location(a, b); a b location.

60 How to create objects? 35/57 int a = 1; int b = 2; Location location = new Location(a, b); a 2 b location.

61 How to create objects? 36/ a 2 b location int a = 1; int b = 2; Location location = new Location(a, b);. 206 Location class. 308 Location object. x y

62 How to create objects? 37/ a 2 b location int a = 1; int b = 2; Location location = new Location(a, b);. 206 Location class. 308 Location object 1 x 2 y.

63 How to create objects? 38/ a 2 b 308 location int a = 1; int b = 2; Location location = new Location(a, b);. 206 Location class. 308 Location object 1 x 2 y.

64 Object creation in memory model 39/57 The first time we encounter a class, we allocate a block in memory for the class. Whenever we encounter new, we allocate a block in memory for the object. Whenever we encounter a constructor, we initialize the attributes by putting the values of the attributes in the block of the object.

65 Terminology 40/57 int a = 1; int b = 2; Location location = new Location(a, b); location is the name of a

66 Terminology 40/57 int a = 1; int b = 2; Location location = new Location(a, b); location is the name of a variable.

67 Terminology 40/57 int a = 1; int b = 2; Location location = new Location(a, b); location is the name of a variable. the type of the variable location is

68 Terminology 40/57 int a = 1; int b = 2; Location location = new Location(a, b); location is the name of a variable. the type of the variable location is Location.

69 Terminology 40/57 int a = 1; int b = 2; Location location = new Location(a, b); location is the name of a variable. the type of the variable location is Location. location is also called an object reference.

70 Types 41/57 We distinguish between primitive types: boolean, char, double, int, long, (byte, float, short) and reference types: classes

71 Invoking a method 42/57 Consider the method public type methodname(type 1 parametername 1,..., type n parametername n ) in the class ClassName. This method is invoked as objectreference.methodname(argument 1,..., argument n ) where the type of objectreference is ClassName and argument i is (compatible with) type i.

72 Location API 43/57 The API of the Location class can be found at the URL /api/location.api/franck/Location.html

73 Invoking a method 44/57 How do you create a Location object named first located at (1,2) and get the location north of it and name that object second?

74 Invoking a method 44/57 How do you create a Location object named first located at (1,2) and get the location north of it and name that object second? Answer Location first = new Location(1, 2); Location second = first. north();

75 Invoking a method 45/57 Draw the diagram representing the memory once the execution has reached the end of the following snippet. Location first = new Location(1, 2); Location second = first. north();

76 Answer 46/ first 408 second. 206 Location class. 308 Location object 1 x 2 y. 408 Location object 1 x 1 y.

77 Printing a location 47/57 Is there a method we can use to print the result?

78 Printing a location 47/57 Is there a method we can use to print the result? Answer Yes, public String tostring()

79 Printing a location 47/57 Is there a method we can use to print the result? Answer Yes, public String tostring() How do we invoke this method?

80 Printing a location 47/57 Is there a method we can use to print the result? Answer Yes, public String tostring() How do we invoke this method? Answer String result = location.tostring()

81 Printing a location 48/57 Location first = new Location(1, 2); Location second = first.north(); String result = second.tostring(); output.println(result);

82 Printing a location 49/57 Exercise Draw the diagram representing the memory once the execution has reached the end of the snippet on the previous slide.

83 Solution to exercise. 534 String class 682 String object (1,1) 50/ first 408 second 682 result. 206 Location class. 308 Location object 1 x 2 y. 408 Location object 1 x 1 y

84 ... objects versus object references 51/57 Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; At the end of the execution of the above snippet, how many objects are there and how many objects references are there?

85 ... objects versus object references 51/57 Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; At the end of the execution of the above snippet, how many objects are there and how many objects references are there? Answer Four objects and six object references.

86 ... objects versus object references Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; At the end of the execution of the above snippet, how many objects are there and how many objects references are there? Answer Four objects and six object references. Exercise Draw the diagram representing the memory once the execution has reached the end of the above snippet. 51/57

87 Solution to exercise 52/ f 400 g 500 h 600 i 400 j 400 k 200 Location class 300 Location object 0 x 0 y 400 Location object 0 x 0 y 500 Location object 1 x 2 y 600 Location object 0 x 2 y

88 When are two objects references the same? 53/57 What do we mean by the same? Do they refer to the same object, that is, do they have the same identity? Do they refer to objects with the same state, that is, do their attributes have the same values?

89 When are two objects references the same? 53/57 What do we mean by the same? Do they refer to the same object, that is, do they have the same identity? Do they refer to objects with the same state, that is, do their attributes have the same values? Location one =... Location other = new Location(1, 1); boolean identical = (one == other); boolean same = one.equals(other);

90 When are two objects references the same? 54/57 Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; Fill the following table with true (T) and false (F). == f g h i j k f g h i j k

91 When are two objects references the same? 55/57 Answer Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; == f g h i j k f T F F F F F g F T F F T T h F F T F F F i F F F T F F j F T F F T T k F T F F T T

92 When are two objects references the same? 56/57 Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; Fill the following table with true (T) and false (F). equals f g h i j k f g h i j k

93 When are two objects references the same? 57/57 Answer Location f = new Location(); Location g = new Location(); Location h = new Location(1, 2); Location i = new Location(0, 2); Location j = g; Location k = j; equals f g h i j k f T T F F T T g T T F F T T h F F T F F F i F F F T F F j T T F F T T k T T F F T T